No Mac in history has been locked down in the way you describe, and there's really no indication that Apple would start now. If they were ever going to, the ARM transition would've been the perfect time to do it, yet they invested engineering resources into adding support for booting non-Apple kernels into their bootloader.
They could of course release a new line of laptops or a firmware update tomorrow which locks down the bootloader and prevents booting non-Apple kernels. But so could Lenovo, Dell, HP, Samsung, Sony, or any other laptop vendor. Or Microsoft, Intel, AMD or Qualcomm could exert their influence, as owners of various parts of the ecosystem, to shift the PC landscape in that direction.
For example, intel and AMD contribute a lot of code and engineering hours to open source projects because they WANT people to be able to run that software on their hardware.
But if you truly do believe that's a good argument, consider Microsoft's position. They wouldn't want you to run non-Windows operating systems and hold considerable power over the Windows PC ecosystem.
